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What happens if the water/powder ratio is altered during alginate mixing?
Which is the recommended mixing time for fast-set alginates?
How long should a typical alginate impression be left in place for optimal tear strength and elastic recovery?
What is the effect of using water that is cooler than 18°C during alginate mixing?
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Which method is recommended to lengthen the setting time of alginate?
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What is the typical elasticity percentage for most alginates?
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What is the risk of undermixing or overmixing alginate?
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What is a significant benefit of providing adequate time for recovery after taking an alginate impression?
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What is the phenomenon called when the viscosity of impression materials decreases with increased shear force?
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What happens to the viscosity of polyether impression materials when the shearing force is removed?
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What primarily influences the shear-thinning properties of polyether impression materials?
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How does temperature affect the working and setting times of elastomeric impression materials?
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What causes dimensional change in impression materials during setting?
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What term describes the behavior of polyether impression materials where they transition sharply into the setting phase?
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What can cause an impression to expand after setting has occurred?
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What is likely to happen if an impression material does not recover elastically after being removed from undercuts?
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What is the compressive strength range of alginate impressions?
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Why is tear strength considered more important than compressive strength in alginates?
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What must be done to an alginate impression before preparing a gypsum model?
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What happens to alginate impressions left out for 30 minutes?
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How should alginate impressions be stored if not used immediately?
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What is the primary risk associated with viruses in dental impressions?
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What is a common method for disinfecting alginate impressions?
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What is the effect of saliva and blood on gypsum setting?
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What is the approximate contraction percentage of addition silicones?
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Which elastomeric impression material has the best elastic recovery during removal from the mouth?
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How does the strain in compression under a stress of 0.1 MPa relate to the material's flexibility?
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Which statement about the hardness of addition silicones is correct?
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Which material is considered to be the least flexible?
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What aspect of a material does tear strength indicate?
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How can low flexibility and high hardness in impression materials be clinically compensated?
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What technique is used to assess the wettability of elastomeric impression materials?
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What is the primary factor that affects the accuracy of dental impressions?
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Which impression material produces slightly better detail even under wet conditions?
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What technique can compensate for dimensional changes on setting of elastomeric impressions?
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What is the effect of inadequate mixing of impression materials?
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What is the recommended handling time for elastomeric impression materials?
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Which method produces fewer bubbles in impression materials?
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What happens to the accuracy of second pours of gypsum products into elastomeric impressions?
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How does the recovery period affect elastomeric impressions post-removal?
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Study Notes
Proportioning and Mixing
- Consistency and setting times of alginate are impacted by water-powder ratio.
- Regular alginate mixing time: 1 minute, fast-set alginate mixing time: 45 seconds.
- Over-mixing and under-mixing negatively affect the strength of the impression.
- Working time: 45 seconds for fast-set (30-75 seconds before setting) and 60 seconds for regular-set (2-3.5 minutes before setting).
- Use mechanical mixing devices or mix vigorously in a flexible rubber bowl with an alginate spatula.
- Color change is a pH-related change of a dye.
Setting Time
- Ranges from 1 to 5 minutes (at least 15 seconds longer than the stated working time).
- Lowering water temperature is preferable to reducing powder to water ratio to lengthen setting time.
- Reducing powder to water ratio decreases strength and accuracy of the alginate.
- The setting reaction is a chemical reaction that doubles in rate with a 10°C increase in temperature.
- Water temperature between 18°C and 24°C is recommended.
Elastic Recovery
- Alginate impressions compress about 10% in undercut areas during removal.
- Permanent deformation is time-dependent.
- Leave the impression in place for 2-3 minutes to increase tear strength and elastic recovery.
Clinical Considerations
- A reasonable bulk of alginate between the tray and teeth, appropriate retention, and rapid removal from the mouth are necessary for a good impression.
- Disinfect the impression and produce a gypsum model to allow for recovery.
Flexibility
- Alginates have a typical flexibility of 14%.
- Hard-set materials have flexibility ranging from 5% to 8%.
- Flexibility is essential for easy removal of the impression.
Strength
- Compressive strength: ranges from 0.5 to 0.9 MPa.
- Tear strength: ranges from 0.4 to 0.7 kN/m; more important than compressive strength.
- Tear strength is a measure of the force/thickness ratio needed to initiate and continue tearing.
- Tearing is more likely in thin areas of an impression and decreases with faster removal.
- Both compressive and tear strengths are time-dependent properties.
Compatibility with Gypsum
- Rinse the impression with cold water to remove saliva and blood, then disinfect.
- Remove surface water before preparing a gypsum model.
- Saliva and blood interfere with gypsum setting, and free water dilutes the gypsum model.
- If the alginate impression is stored for longer than 30 minutes, rinse with cool water to remove exudate and wrap in a moist paper towel sealed in a plastic bag.
Dimensional Stability
- Alginate impressions lose water by evaporation and shrink when exposed to air.
- Impressions left out for as short as 30 minutes can become inaccurate.
- Pour the model material into the alginate impression as soon as possible for optimal accuracy.
- If model preparation is delayed, store impressions in 100% relative humidity in a plastic bag or damp paper towel.
Disinfection
- Viruses can be transferred to gypsum models and present a risk.
- Spraying is the most common disinfection method, but immersion in disinfectant is also effective.
Shear Thinning
- Shear force affects viscosity of poly ether and addition silicone impression materials.
- Shear thinning (or pseudo-plasticity) reduces viscosity of the unset material with increasing outside force or shearing speed.
- Viscosity increases when the force is discontinued.
- This property is important for monophase impression materials.
- In polyether, shear thinning is influenced by triglyceride crystals which align when the material is sheared.
- The triglyceride network keeps the polyether viscous in the tray but allows it to flow under pressure.
- Cooling increases viscosity; bring pastes to room temperature before use.
Working and Setting Times
- Working and setting times for elastomeric impression materials decrease as viscosity increases.
- Polyethers have a defined working time with a sharp transition to setting, known as snap-set.
- Working and setting times are shortened by increased temperature and humidity.
Dimensional Change on Setting
- Elastomeric impression materials undergo dimensional change on setting.
- Contraction during setting is due to cross-linking and bond rearrangement.
- Expansion can occur if water sorption takes place.
- Distortion can occur if the material is seated after partial setting.
- Creep can occur if the material does not recover elastically after removal from undercuts.
Mechanical Properties
- Addition silicone and polyether impression materials undergo shrinkage due to polymerization.
- Addition silicones have the smallest change (~0.15%) followed by polyethers (~0.2%).
- Contraction is low due to minimal byproduct loss.
- Most shrinkage occurs in the first hour after removal.
- Prepare models promptly for maximum accuracy.
- Elastomeric impression materials are more stable in air than hydrocolloid products.
Elastic Recovery
- Addition silicones have the best elastic recovery followed by polyethers.
Strain in Compression
- Strain in compression under 0.1 MPa stress measures material flexibility.
- Polyethers are the stiffest, followed by addition silicones.
Flow
- Flow is determined 15 minutes after applying a 1 N load to a 1-hour-old cylindrical specimen.
- Silicones and polyethers have low flow values.
Hardness
- Hardness increases from low to high consistency.
- Addition silicones maintain relatively consistent hardness over time, while polyether hardness increases with time.
Tear Strength
- Hardness and strain in compression influence the force needed for removal.
- Low flexibility and high hardness can be compensated by increasing impression material space.
- Tear strength indicates the material's ability to resist tearing in thin areas and margins of periodontally involved teeth.
- As consistency increases, tear strength slightly increases.
- Polysulfide is the most flexible, and polyether is the least flexible.
Wettability and Hydrophilization
- Wettability is measured by advancing contact angle of water on the set impression material.
- Tradition addition silicone is less wettable than polyether.
- Wettability and hydrophilization affect detail reproduction and bubble formation when pouring gypsum.
- Most impression materials produce acceptable detail under wet and dry conditions.
- Polyethers produce slightly better detail than addition silicones and are unaffected by moisture.
Relationship of Properties and Clinical Application
- Accuracy, detail recording, handling ease, and setting characteristics are crucial in dental impressions.
- Placement timing is critical as viscosity increases rapidly with time.
- Thorough mixing prevents incomplete polymerization and inconsistent setting.
- Auto mixing and mechanical mixing systems reduce bubbles and improve accuracy.
- Manufacturers recommend leaving the impression in the mouth for a minimum time.
- Dimensional changes can be compensated for with double-impression or putty-wash techniques.
- A custom-made tray enhances accuracy with monophase and simultaneous dual-viscosity techniques.
- Viscosity is the most important factor for minimizing bubbles and maximizing detail.
- The amount of deformation and removal time can affect impression accuracy.
- Second gypsum pours produce less accurate dies than the first.
- Elastomeric impressions continue to recover for a period after removal, increasing accuracy over time.
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Description
This quiz covers the essential aspects of mixing and setting times for alginate in dental practices. Understand the impact of water-powder ratios, standard mixing times, and the importance of maintaining the right consistency. Additionally, explore how temperature affects the setting reaction for more accurate impressions.
